This invention relates to a nuclear fuel rod for use in a nuclear reactor, and more particularly to a novel supporting structure used in such nuclear fuel rod to accomodate the stack length of fuel of pellets in a cladding or sheathing tube.
In fabricating the nuclear fuel rod of the type used heretofore, nuclear fuel pellets are charged into a cladding or sheathing tube (usually 7 to 17mm in outer diameter, 0.4 to 1.2mm in thickness and about 4m in full length) and then end plugs are welded to both ends of said cladding tube for hermetically sealing, with a coil spring being interposed between the upper end plug and the pellet stack. The upper space occupied by such coil spring, which is a plenum, proves effective in absorbing thermal expansion of the pellet stack or internal pressure built up in the cladding tube by the gas generated during nuclear fission, while the coil spring per se provides a pressing force against the pellet stack to prevent shaking or other un-favourable movement of the pellets during handling of the fuel rod before it is charged into the nuclear reactor.
It is a general practice that the clearance between the cladding tube and the pellets is less than about 0.3mm at the time of fabricating the fuel rod. However, such clearance tends to be reduced under the irradiation for several reasons such as that the pellet temperature becomes far higher than that of the cladding tube, and that the pellets have a greater degree of thermal expansion than the cladding tube. Further, cracks or flaws could develop in the pellets at the initial stage of reactor operation and the pellets might be strongly adhered at least partially with the cladding tube. Therefore, pellets are not always freely movable longitudinally in the cladding tube. In such cases it is hardly possible for the upper end space alone to sufficiently absorb the axial movement of the pellet stack caused by thermal expansion of the pellets.
The heat generation in the nuclear rod is higher in its middle portion than in the other portions, so that it can be expected that the strong mechanical interaction may occur between the pellets and the middle portion of the cladding tube. During such occasions as restarting of the reactor, over-power operation or rapid increase of reactor output, it could be difficult to sufficiently absorb thermal expansion of the pellet stack located in the lower-half portion adjacent to the lower end plug, with the result that excess stress is given to the cladding tube or the lower end plug joint which could cause a rupture of the fuel rod. In order to solve such problem, it has been attempted to provide a coil spring and a space not only at the top of the pellet stack but also at the bottom thereof, i.e. between the bottom end of the pellet stack and the lower end plug, to thereby absorb downward elongation of the pellet stack. However, such support using a coil spring has a disadvantage that spring property could be changed or relaxed during use of the reactor, making it impossible to maintain the original bottom position of the pellet stack. If the bottom of the pellet stack is lowered, voids are formed between the pellets in the stack which cause local elevation of the fuel rod output (forming so-called hot spots) which lead to an undesirable performance of the safety of the nuclear fuel.